Will Thunderbird 52 still receive security updates only, for XP users when Thunderbird 53 arrives? Or would XP users be moved to an ESR Thunderbird version (like what is happening with Firefox)? Any significant dates here?

There aren't dedicated ESR releases for Thunderbird anymore. Starting with version 55 each cycle (nightly/aurora/beta/release) is going to be shortened by 6 weeks so major versions will be released more quickly. I suspect they have not made any decision yet what to do about XP security patches. Things are also complicated by how they plan to deal with Gecko (the layout/rendering engine from Firefox) dropping support for XUL based add-ons in version 57, and the Firefox developers having been told not to waste any time preventing problems for Thunderbird (the so called Thunderbird tax). With each release it probably gets more expensive for the Thunderbird developers to find and deal with those problems. Its not obvious what version of Gecko will be used in the next major version of Thunderbird as they are still working on a long term plan to provide technical independence from Firefox/Gecko.